Field of Invention
The present invention relates generally to monitoring impairments in a CATV or HFC network, and more particularly methods and apparatus for pinpointing nonlinear impairments such as common path distortion (CPD) and passive intermodulation distortion (PIM) in a modern CATV or HFC network transmitting OFDM signals.
Background Art
One type of impairment in a modern hybrid fiber coax (HFC) network is common path distortion (CPD). This impairment is nonlinear and involves the intermodulation of forward path (or down stream) signals occurring at various network components in the “common path” (i.e., both forward and return paths) of the network, such as amplifiers, connectors, taps, terminations, etc. The intermodulation of the forward path signals is the result of a so-called “diode effect” caused by corrosion of the above-mentioned components in the network. A very good explanation of the causes of CPD is found in an article by Bharat (Barry) Patel, entitled, “Report on Common Path Distortions” or “Characterization of Common Path Distortions,” dated Feb. 3, 1998 (http://cable.doit.wisc.edu/cpd/cpd2.pdf or http://www.arcomlabs.com/images/documents/cpd_report.pdf). CPD produces composite triple beat (CTB) and composite second order (CSO) distortion products in both the upstream and downstream spectrums (return and forward paths) of the HFC network. In addition,the locations of CPD sources are often locations where noise is introduced into the CATV network (i.e., ingress points). As explained in a presentation entitled, “RF Impairments in the Return Path & Their Impact on a DOCSIS Performance or the RF World according to Jack,” by Jack Moran, Motorola, dated Sep. 23, 2003, CPD influence is one of the most important impairment problems affecting the quality of return path service. Accordingly, the task of finding and eliminating CPD is an important one to ensure quality of service in both the upstream and downstream spectrums. However, this task is a challenge due to the hundreds and sometimes thousands of nodes, network components, and miles of network cable in a network.
More specifically, with respect to modern HFC networks, the detection and location of CPD is important, because CPD impacts data transmission particularly in the upstream frequency spectrum. For many years now, network impairment statistics have been collected by different monitoring systems of cable operators, and such statistics have convincingly established that CPD exists in substantially all HFC networks and that the CPD problem increases as such networks age and corrosion of network components becomes more prevalent.
In recently specified and implemented HFC networks (“modern HFC networks”), the CPD impairment problem is expected to become more difficult to solve due to several factors. First, the number of quadrature amplitude modulated (QAM) signals in the upstream spectrum (or return path) will increase due to channel bonding technology specified in the Data-Over-Cable Service Interface Specifications (DOCSIS) 3.0 specification, published in August 2006 by Cable Television Laboratories, Inc. (CableLabs®) of Louisville, Colo. As a result, the level of each QAM signal will be reduced and the immunity of these signals to noise (in the return path) will be reduced as well. Second, there is a migration away from analog channels to all digital channels in the downstream spectrum (or forward path). In the case of analog channels, CPD generally appears at harmonics of 6 MHz and is thus relatively easy to detect, while, in the case of QAM or other digital channels, CPD appears as flat noise spread over the full upstream spectrum, making it more difficult to detect. Third, under the very recent DOCSIS 3.1 specification (October 2013), the bandwidth of both the downstream and upstream spectrums in HFC networks will increase. For example, the upstream spectrum bandwidth may increase to 204 MHz and the downstream spectrum bandwidth may increase to 1794 MHz. With these increases in bandwidth, the number of different order nonlinear intermodulation (IM) products generated at a CPD source will increase, which may impact data signals at least in the upstream spectrum.
There are basically two known methods of detecting CPD. The first is to use a spectrum analyzer to monitor the upstream and/or downstream spectrums for IM products, presumably caused by CPD. Such a method is adequate for legacy HFC networks carrying a large number of analog channels. In these networks, CPD looks like a number of discrete 6 MHz harmonics (for NTSC frequency plan). But, in the case of an all-digital network (e.g., QAM and/or OFDM signals), CPD in the upstream spectrum looks like flat noise and is not easily distinguishable from additive ingress noise. Another limitation is that a spectrum analyzer does not allow one to identify multiple sources of CPD, which is a typical impairment scenario. Also, a CPD source cannot be directly located using a spectrum analyzer. Further, a spectrum analyzer is unable to detect very low level CPD distortion products. The ability to detect very low level CPD is desirable because it allows one to identify CPD sources early in their development, before they impact signal quality, thus making it possible to implement a preventative or proactive network maintenance program. Also, very low level CPD detection is useful to identify CPD sources that may currently impact signal quality, but are intermittent (which has been shown to be a common behavior). In such case, low level CPD may increase dramatically for a moment due to mechanical (e.g., wind), temperature, moisture and other environmental factors.
The second known method of detecting CPD is known as the Hunter® Xcor® system available from Arcom Digital, LLC, Syracuse, N.Y. (http://www.arcomlabs.com/4HunterPlatform.html). This system is described in the following patent documents: U.S. Pat. No. 7,415,367 to Williams and U.S. Pat. No. 7,584,496 to Zinevich. The Hunter® Xcor® system is sometimes referred to as a CPD radar or a nonlinear radar system. The main idea behind the system is to use regular, commercial traffic, downstream signals as a probe signal and examine the so-called “echo” of the probe signal (or “echo signal”), which, due to the non-linearity of the CPD impairment, is returned in the upstream spectrum. The echo signal comprises the IM products of the downstream signals (e.g., QAM signals) and is obviously delayed relative to the original downstream signals. The echo signal is artificially duplicated (or emulated), with a zero relative delay, at or near the origin of the downstream QAM signals. The zero-delay echo signal is created by mixing the downstream signals at a diode or other non-linear (active or passive) device. The resulting zero-delay echo signal is then sampled to produce reference samples. The reference samples are cross-correlated with samples of signals in the upstream spectrum, which include the actual echo signal (e.g., second and third order IM products). The cross-correlation output indicates a detection of the CPD source by a correlation peak and the position of the peak reveals a time delay associated with the round-trip propagation time of the downstream signals (probe signal) and the upstream CPD IM products (echo signal). From this time delay, and the use of a network map or time delay database, the location of the CPD can be found. The main advantages of this system are its ability to detect low level CPD and multiple CPD sources, due to good sensitivity and time delay resolution. This technology is now widely used in HFC networks across the United States and in many other countries. It has proven to be very effective in the early detection and location of CPD.
In a modern HFC network with a Converged Cable Access Platform (CCAP), as currently being specified in the DOCSIS 3.1 specification, there will be an increase in narrowcast signals in the downstream spectrum. For example, according to DOCSIS 3.1, the next generation cable modem termination systems (CMTS's) will form very wideband (e.g., up to 192 MHz) orthogonal frequency division multiplexing (OFDM) narrowcast signals. Narrowcast signals are (by name) tailored for narrower audiences and different packages of narrowcast signals are served to different audiences, in different nodes, from different CMTS's. Thus, a common and full downstream spectrum of signals for use as a probe signal (as found in current or legacy networks) will not generally be available or possible in the recently specified networks. Also, emulation of the echo signal (CPD IM products) has traditionally been accomplished using an analog diode or mixer. Such devices may not accurately emulate high order IM products, expected with OFDM signals. Further, for optimum CPD detection, a vacant part of the upstream spectrum should be used, because the relatively high level of upstream QAM data carriers (formed by cable modems) may interfere with typically low level CPD echo signals. Generally, the vacant part of the upstream spectrum is 5 to 20 MHz, but this part of the spectrum is noisy due to ingress. Ingress can limit the sensitivity of the CPD radar. If the bandwidth of the CPD radar is limited to minimize ingress, the limited bandwidth will limit the sensitivity and time delay resolution of the CPD radar. Moreover, in a CCAP network architecture, downstream signals used to emulate the CPD echo signal will have to be obtained from multiple CMTS's (because each CMTS may produce a relatively unique package of downstream narrowcast signals). This could require extra equipment (e.g., multiple CPD radar units and return path switches) to be installed at the headend of the network or at distributed points between the headend and the fiber nodes. Such extra equipment would require extra space and power consumption. In some cases, for example, in a Fiber Deep system proposed by Aurora Networks, Santa Clara, Calif. (www.aurora.com), where the CMTS card is installed directly at the fiber node, convenient connection to upstream test points to receive CPD echo signals may be impossible.
According to the CableLabs® Proactive Network Maintenance (PNM) specification (http://www.cablelabs.com/specification/proactive-network-maintenance-using-pre-equalization/), entitled DOCSIS® Best Practices and Guidelines, Proactive Network Maintenance Using Pre-Equalization, CM-GL-PNMP-V02-110623, Jun. 23, 2011, the detection of any impairment in an HFC network is preferably performed at the CMTS or cable modem (CM): “The CMTS and CM contain test points which include essential functions of a spectrum analyzer, vector signal analyzer (VSA), and network analyzer, while the cable plant is considered the Device Under Test (DUT). The goal is to rapidly and accurately characterize, maintain and troubleshoot the upstream and downstream cable plant, in order to guarantee the highest throughput and reliability of service.” The concept of using CMTS's and CMs for network maintenance is very reasonable, cost effective, and may be the best solution for HFC networks with CCAP architecture.
Detection methods of nonlinear distortions in the downstream and upstream spectrums have been recently proposed and are described in the following CableLabs® documents: (1) Testing Nonlinear Distortion in Cable Networks, by Thomas Williams and Belal Hamzeh, CableLabs, 2013; (2) Field Measurements of Nonlinear Distortion in Digital Cable Plants, by Thomas Williams and Belal Hamzeh, CableLabs, Ronald Hranac, Cisco Systems, 2013; (3) Cable Network Management Infrastructure Evolution, by L. Alberto Campos, Jennifer Andreoli-Fang and Vivek Ganti, CableLabs, 2014; and (4) Proactive Network Maintenance Using Pre-Equalization, DOCSIS® Best Practices and Guidelines, CM-GL-PNMP-VO2-110623, CableLabs 2011. The first two documents describe a method of detecting downstream nonlinear distortions by first capturing samples of the full downstream spectrum at CMs and using the samples to emulate (or “manufacture”) second and third order CPD distortion signals and then cross-correlating the emulated distortion signals with a portion of the downstream spectrum (presumably containing actual CPD distortion signals) in a “vacant” band (i.e., no downstream commercial signal traffic). The second and third order CPD distortion signals are known as composite second order (CSO) and composite triple beat (CTB), respectively. Theoretically, the method of the first two documents can be used to predict the CPD condition in the upstream spectrum as well, because second and third order CPD distortion affect both downstream and upstream spectrums (i.e., high and low frequency IM products). However, in reality, it is not so simple. For instance, such a predictive method does not yield an actual CPD level in the upstream spectrum, because the relative CPD level in the upstream depends on the upstream signal level at the CPD source. For example, very often CPD appears at the output of an amplifier or a nearby passive, where the downstream signal level is high. At these points, a relatively low level second order IM product in the downstream may result in a relatively high CPD level in the upstream, because the upstream signal level is generally at a minimum at these points.
Also, with respect to the above predictive method, the level of second order IM products in the vacant downstream detection band do not, generally, correlate with the level of the second order IM products in the upstream, because the second order IM products in the downstream include harmonics of the downstream carriers (2f1, 2f2 . . . 2fn), while CPD in the upstream is formed from IM difference products between downstream carriers (f2−f1) only. Further, the level of third order IM products in the downstream cannot be used as a reliable indicator of CPD, because third order IM is generated at all amplifiers and only those amplifiers with, e.g., a poor performing diplex filter will produce CPD. Thus, in general, one cannot distinguish, among all amplifiers, which one is the CPD source. Lastly, the above predictive method does not directly identify a correct number and location of the CPD sources in the node.
The second CableLabs® document listed above also describes a method of detecting nonlinear distortion in the upstream spectrum from upstream signals. The idea is to use a statistical correlation of energy in a vacant upstream band during transmission of upstream burst signals. This method detects nonlinear distortions of upstream signals, whether or not those distortions are produced by a CPD source. That is, the method does not distinguish between nonlinear distortions from, e.g., CPD and amplifiers operating in a nonlinear region. Also, due to a higher signal level in the downstream (compared to upstream) and a much higher total energy of the downstream signals, the level, probability, and impact of CPD distortion in the upstream from downstream signals are often higher than of nonlinear distortions (generally) in the upstream from upstream signals.
Referring now to the third and fourth CableLabs® documents listed above, they describe a method of using pre-equalization between CMs and CMTS's, for detecting and locating linear distortions. This method is part of a scheme called Proactive Network Maintenance (PNM), and is addressed in detail in the fourth CableLabs® document. The fourth document mentions that even impairments considered “nonlinear” such as CPD may have associated linear distortion elements. This is true in the sense that a nonlinear element, such as a corroded center conductor, may also create an impedance mismatch, which may generate a micro-reflection (a linear distortion). But, relying on whether a nonlinear element also has a linear distortion associated with it, is an indirect and imperfect assessment. For example, a saturated amplifier, operating in a nonlinear region, generally would not have a linear distortion element associated with it. A pre-equalization method does make sense as a second step, after CPD source candidates are identified in a first, more direct step. For instance, the pre-equalization method may be used in a second step to reduce the number of CPD candidates identified in the first step (e.g., those candidates that do not have a linear component, as determined under the second step, may be eliminated as a potential CPD source).